This invention relates to methods and devices for weighing. In particular, the invention relates to rapid and accurate weighing. In one aspect, the invention relates to accurate and rapid weighing of relatively small items, in the 0.1 to 10 gram range, such as nuclear fuel pellets and pharmaceutical tablets and capsules, at speeds of up to 100 items per second and above.
The invention relates to subject matter described in Disclosure Document No. 059719, filed Apr. 14, 1977.
Many attempts have been made in the past to achieve rapid and accurate weighing. However, almost all systems for weighing individual items in a dynamic way have involved placing the part onto a weigh pan, observing its deflection, and then removing the item, all at the same station. Because of the mechanical motions involved, a limited speed results if accuracy is to be maintained. On the other hand, on- and off-loading functions preclude accuracy. Because of these limitations, existing pharmaceutical scales operating in this manner cannot weigh more than about 30 tablets per minute if an accuracy of 1 milligram per gram (0.1%) is to be achieved. Accordingly, in the pharmaceutical industry, tablets and the like are not generally one hundred percent weighed.
Where lesser accuracy can be tolerated, higher speeds are attainable. For example, the items to be weighed can be loaded onto a belt which in turn passes over a weigh pan dynamically. The accuracies claimed for these devices are generally 0.5 to 1% but actual results are often considerably worse.
The prior art literature relating to accurate high speed weighing is, of course, extensive. In addition to the devices mentioned above, various other systems have been suggested. All of these systems seek the same results: speed and accuracy. However, insofar as we are aware, no previously proposed system is even theoretically capable of achieving a high degree of accuracy and speed without sacrifice of some other important factors, such as reliability, durability, expense, or the like.
It is an object of the present invention to provide a rapid and accurate weighing method and apparatus. It is a further object of the invention to provide such method and apparatus which is nevertheless inexpensive, reliable, and durable.